Circuit for generating a saw-toothlike current in the deflecting coil of cathode-raytubes



May 31,1949. KERKHQF 2,471,819

CIRCUIT FOR GENERATING SAWTOOTH-LIKE GURRENTS IN THE DEFLECTING COILS OF GATHODE-RAY TUBES Filed April 12, 1946 2 Sheets-Sheet 1 y 1949- F. KERKHOF 2,471,819

\ CIRCUIT FOR GENERATING SAWTOOTH-LIKE CURRENTS IN .THE .DEFLECTING COILS OF CATHODE-RAY TUBES File d April 12, 1946 1 2 Sheets-SheetZ A77 ORA/1F Y Patented May 31, 1949 CIRCUIT FOR GENERATING A SAW-TOOTH- LIKE CURRENT IN THE DEFLECTING COIL F CATHODE-RAY TUBES Frederik Kerkhof, Eindhoven, Netherlands, as-

signor, by mesne assignments, to Hartford National Bank and Tru Conn., as trustee st Company, Hartford,

Application April 12, 1946, Serial No. 661,527 In the Netherlands November 7, 1945 3 Claims.

This invention relates to a circuit for generating a sawtooth-like current in the deflecting coils of a cathode ray tube, particularly in the image deflecting coils of the cathode ray tube in a television device.

A circuit of this kind generally includes a source that generates a sawtooth voltage and an amplifier valve into whose anode circuit the deflecting coils are included via a transformer. If the sawtooth-like voltage is supplied to the control grid of the amplifier valve directly, the curve shape of the current passing through the deflecting coils is found to diverge from the sawtooth-shape in that during the stroke of the sawtooth the current does not increase lineally as a function of time, as in the case of a pure sawtooth-course. The lower the frequency of the sawtooth oscillation and the smaller the primary inductance of the transformer, the more intense is this distortion. That is why this transformer-coupled circuit is not serviceable without additional means for the image deflection in a television device in which the frequency of the sawtooth oscillation is 50 or 60 cycles/sec.

The distortion could be avoided by choosing a very high induction of the primary winding but this leads to transformers of large size, which are expensive, so that this solution is not very suitable in practice. It has been suggested to suppress the distortion by the use of a negative feedback, for example by providing the transformer with a negative feedback winding, which is included in the grid circuit of the amplifier Valve, so that a negative feedback voltage is effective in the grid circuit and, jointly with the sawtooth voltage, controls the grid. Instead of using such a negative voltage feedback it is more favourable to use a negative current feedback, a voltage proportioned to the current that passes through the deflecting coils being negatively backcoupled to the grid circuit of the amplifier valve.

It is also possible to suppress the distortion by feeding the sawtooth voltage via a phaseshifting network to the control grid of the amplifier valve, said network bringing about a phase displacement substantially opposite to the phase displacement brought about by the transformer. The two solutions just mentioned have this feature in common that a voltage counteracting the distortion is introduced into the grid circuit of the amplifier valve and, as found on closer examination, this voltage must be a parabolic function of time. In this case, the voltage effective between control grid and cathode is constituted by a sawtooth and a parabolic voltage and the more intense the distortion, or in other Words the lower the inductance of the primary Winding of the transformer, the greater has to be the amplitude of the latter voltage. As the primary inductance decreases, the shape of the anode current of the amplifier valve will start to diverge more and more from the sawtooth-shape. Incidental to it is a variation of the anode direct current.

According to the invention the primary inductance of the transformer is chosen to be such that the anode direct current of the amplifier valve is at least approximately a minimum.

The invention is consequently based on recognition of the fact that there is a given value of the primary inductance at which the parabolic voltage at the control grid that counteracts the distortion brings about such a divergence of the anode current from the sawtooth-shape that the anode direct current of the amplifier valve becomes a minimum.

In order that the invention may be clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying drawing.

The circuit shown in Fig. 1, comprises an amplifier valve I of the pentode type, in the anode circuit of which the deflecting coils 2 of a cathode ray tube (not shown) are included via a transformer 3.

Fig. 2 shows the substitution diagram of the transformer and the deflecting coils, Lp designating the primary inductance of the transformer and Ls and Re the inductance and resistance of the deflecting coils as transformed on the primary side.

Between the current 23; passing through the primary winding and the current s, transformed on the primary side and passing through the defleeting coils, obtains the differential equation L,%=R,i,+L.%-: 1 The desired sawtooth-like course of the current is is indicated in Fig. 3 and in the time interval of one cycle this can be represented by T being the cycle of the sawtooth oscillations. By substituting 2 in 1 and integrating:

2 a= t)i..+0

9 The integration constant 0 contained therein may L,, 1 TR,

The anode current is. thus becomes:

. a 2 L 1 TR, 2t

and this shows that the current is composed of lineally increasing current (the terms containing the first power of t), a parabolic current (the term containing the second power of t) and a direct current iao which. is given by Fig. i is a graph showing the relation between 52.0 and Lp as given by this equation and shows that as the primary inductance Lp decreases the anode direct current falls and on having reached a minimum at increases again.

According to the invention, in order to miniraise the energy taken up by the tube i from the anode supply source, the primary inductance of the transformer is chosen to be such that the relation (7) is fulfilled at least approximately, the circuit thus acting with a maximum output.

The curve-shape of the anode current which is thus set up and which is found b substituting (7) in (5) is shown in Fig. 5.

If use is made of a rectifier valve having a high internal resistance, the voltage supplied to the control grid mustv have the same shape. If starting with a sawtooth-like voltage this may, as already mentioned hereinbefore, be ensured by either using a negative feedback or by feeding a sawtooth-like voltage via a, phase shifting network to the control grid, as shown in Figs. 6 and '7 respectively.

Fig. 6 shows a negative feedback circuit, a resistance, which is also included in the grid circuit of the amplifier, I, being connected in series with the deflecting coils. The source of voltage that supplies a sawtooth-like voltage is designated by 5.

In the embodiment shown in Fig. 7, the sawtooth-like voltage, which is generated by the source 5, is fed by a phase shifting network constituted by condensers 6 and I and a resistance 8 to the control grid of the amplifier valve l.

What I claim is:

1. A circuit for generating a sawtooth-like current in the deflecting coils of a cathode ray tube, comprising: a vacuum tube amplifier, the anode circuit of which includes a transformer coupling to the deflecting coils; a source of supply of sawtooth voltage connected to the input of said amplifier; means for correcting the distortion of the current in the deflecting coils; and in which circuit, the primary inductance of the transformer is equal to substantially .29'IRs, where T is the period of the saw tooth current and Rs is the transformed secondary resistance, whereby the D. C. component of the anode current is maintamed a minimum.

2. A circuit for generating a sawtooth-like current in the deflecting coils of a cathode ray tube, comprising: a vacuum tube amplifier, the anode circuit of which is transformer-coupled to the deflecting coils; a source of supply of sawtooth voltage which is fed to the input of said amplifier; feedback means for correcting the distortion of the sawtooth wave in the deflecting coils; and in which anode circuit, the primary inductance of the transformer is equal to substantially .29TR5, where T is the period of. the sawtooth current and Rs is the transformed secondary resistance, whereby the D. C. component of the anode current is maintained a minimum.

3. A circuit for generating a sawtooth-like current in the deflecting coils of a cathode ray tube, comprising: a vacuum tube amplifier, the anode circuit of which is transformer coupled to the deflecting coils; a source of supply of sawtoothiike voltage which is fed to the input of said amplifier via a phase shifting network; and in which anode circuit, the primary inductance of the transformer is equal to substantially .29'IRs, where T is the period of the sawtooth current and R5 is the transformed secondary resistance, whereby the D. C. component of the anode current is maintained a minimum.

FREDERIK KERKHOF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,059,219 Farnsworth et al. 1 Nov. 3, 1936 2,284,378 Dome May 26, 1942 2,414,546 Nagel Jan. 21, 1947 2,440,786 Schade May 4;, 1943 FOREIGN PATENTS Number Country Date 117,424 Australia a- Sept. 2, 1943 

